1. Field of Invention
The invention relates to a unitary body plotter pen, and in particular a construction useful as a disposable plotter pen. A writing tube is provided centrally and at the front end of a unitary hard plastic body element that defines a writing tube nib; a housing for a soft plastic pressure equalization chamber and an ink reservoir; and an adaptor structure for mounting with a plotter head. As used hereinafter, the terms "soft" or "softer" plastic connote an elastic deformability that is relatively greater than that of a "hard" or "harder" plastic. The writing tube is fixed axially at the front end of a harder plastic unitary body element and axially connects to a writing ink storage space defined proximate to a rearwardly open, rear end of the body element. An axially extending pressure-equalization chamber is defined below the writing ink storage space by softer plastic vent channel structure that is inserted axially against the inner surface of the body element. The pressure-equalization chamber has a rear end that connects to a central bore communicating with the writing ink storage space and a front end that connects to the ambient air, through an annular area at the front end of the body element. That annular area also may include transverse capillary spaces as a further protection against ink leakage from the vent channel of the pressure equalization chamber.
2. Brief Description of the Prior Art
In the case of one known plotter pen of this type (Germany Off. DE 40 13 510, U.S. Pat. No. 5,172,995), a pressure-equalization chamber has one end that is connected to the writing ink storage space near a front area of the pen; a connection to ambient air that is provided at a rear end of the pen; and a spiral-shaped pressure-equalization chamber that extends from rear to front. The rear end also extends into a space filled with a flexible, open-pored material, such as a foam. This open-pored material exhibits capillary spaces, and is connected to the ambient air by a rearward extending bore. Hence, if there is an inner pressure increase so strong that the entire pressure-equalization chamber is filled with writing ink (due to a sudden temperature increase, for example) then any writing ink reaching the rear end of the pressure-equalization chamber will be sucked up by the open-pored material. There is, therefore, no danger of free writing ink escaping from the pressure-equalization chamber and into the environment. However, once the flexible, open-pored material has sucked up excess writing ink, further through-flow of air may be obstructed, and that condition may bring about a reduction in maximum plotter pen writing speed.